1. Field of the Invention
This invention pertains to the field of screeds for screeding cementious material and more particularly to power-driven screeds.
2. Description of the Related Art
A non-powered roller screed is sold by Bunyan Industries of Salt Lake City, Utah. The roller screed consists of an elongate tube having rotatably mounted handles at its distal ends. The roller screed is used on freshly poured cement by placing it on top of a pair of spaced apart and parallel rails that are set level with the desired surface height for the concrete pad being formed. The roller screed is then pulled along the top of the rails pushing excess cement ahead of it and leaving a flat level surface behind it which may be finished with trowels after the cement has set enough to support the weight of people and tools. Several deficiencies can be readily noted with the rotary screed just described: mainly, being manually operated, it requires two strong operators to pull the screed and move excess cement, and the length of the screed is limited. Screeds over twenty feet long sag in the center and create an uneven cement surface. Thus, it is desirable to create a roller screed that is easier to operate and that can be longer than twenty feet without sagging.
A powered screed is shown in Morrison, U.S. Pat. No. 4,931,008, which discloses a vibrating screed that structurally consists of a ridge beam supported by a plurality of knee braces connected to a T-shaped screed blade and a bull float blade. The individual sections are connected by a modified turnbuckle that can be adjusted to make the screed and bull float blades level. This general concept has been incorporated into screeds sold by M-B-W, Inc., of Slinger, Wis., which claims that its screeds can maintain commercial finishing tolerances at screed lengths up to 60 feet. However, the angle iron screed blades associated with these screeds are less desirable than roller screeds.
A powered roller screed is shown in Garner et al., U.S. Pat. No. 4,964,754, which discloses an operator driven power roller screed having tandem drive rollers and a screed roller. Although Garner et al. overcomes the problems associated with the manual roller screed, the apparatus is large and cumbersome to move into place and must be relatively short, as compared to the Morrison-style screed, to avoid sagging in the middle.